1. Field of the Invention
The present invention relates to fabrication of a glass substrate for a magnetic disk used in a hard disk or the like used as a large scale record medium of a computer, particularly to a method of cleaning a glass substrate for a magnetic disk using a glass substrate pulled up from a chemically strengthening treatment solution produced by an alkaline ion exchange.
2. Description of the Related Art
Although conventionally, a substrate which uses an aluminum alloy and the surface of which is plated with nickel and phosphor has widely been used in recent years as a substrate for a magnetic disk, the demand for a glass substrate has been increasing owing to necessities of small size formation of, a hard disk drive, high recording density of a disk, as well as low flying height of a magnetic head to achieve the high density, promotion of impact resistance and the like in.
Properties of withstanding centrifugal force caused by rotation of a disk drive and impact force caused by collision with a magnetic head and the like are required for a glass substrate for a magnetic disk and a glass substrate having large mechanical strength different from ordinary glass is necessary.
In order to satisfy the above-described mechanical strength, there has been used a glass substrate of crystallized glass having a structure in which fine crystals are dispersed and strength is provided by strain caused by dispersion, or a glass substrate of an ion strengthening type in which after fabricating a glass substrate in a predetermined shape, an alkaline metal (for example, K.sup.+) having a large ionic radius is permeated to a surface of the glass substrate by thermal diffusion which provides large compressive stress caused by a difference in ionic radii at the surface of the glass substrate to thereby strengthen the glass substrate.
Particularly, in recent years with high density recording of memory elements on a hard disk accompanied by using an MR head (magnetic resistance head) and a GMR head (giant magnetic resistance head), in correspondence therewith, it is necessary for the magnetic head to have a smooth face to a degree that a flying height the magnetic head above the surface of the disk substrate falls in a region of lower than 300 A (Angstrom).
However, the size of crystals of the crystallized glass is in the magnitude of a micrometer order and therefore, there has been arisen a problem in which the roughness of the surface which originally is smooth becomes varied in the order of micrometers due to a difference in hardnesses of an amorphous matrix shape and crystals or a difference in chemical properties thereof. This gives rise to a problem in which when such a glass substrate is used in a magnetic disk, so-to-speak low flying height where a head of the MR head or the GMR head or the like is floated up proximate to the magnetic disk is difficult to carry out, or in respect of the head, in the case of so-to-speak narrow track formation where a memory track is formed in a narrow region with high density recording of the magnetic disk, so-to-speak modulation (adjustment node) is observed in reproduced output. In the case of a magnetic disk using the above-described crystallized glass by avoiding such a problem, it is difficult to provide the surface of the disk substrate with predetermined smooth face accuracy.
As a result, at present, use of crystallized glass is shunned in a magnetic disk and strengthened glass produced by alkaline ion exchange has widely been used. However, according to a hard disk using strengthened glass produced by alkaline ion exchange, when a disk is driven in the case where the MR head or the GMR head is used, in respect of a limit in recording density, in view of medium noise, a magnetic memory layer on the surface of the disk substrate is made as thin as below 100 A (Angstrom) and further, a protective film formed on the surface is made as thin as about 50 A (Angstrom) although conventionally, about 150 A (Angstrom) has been needed in order to reduce space loss owing to a medium of the head per se.
Particularly, in respect of the medium noise, factors for determining the medium noise are surface smoothness of a glass substrate and moisture adsorbed to the glass substrate in a step of sputtering a magnetic film. That is, the glass substrate is cleaned immediately before entering a medium fabrication step and when an alkaline component is present on the surface of the glass substrate, moisture is liable to adsorb and the S/N (signal to noise) ratio that is one of medium properties is deteriorated by adsorbed moisture.
Moreover, when an alkaline component is present on the surface of a glass substrate, the alkaline component is made to permeate into a magnetic film formed on the surface of the glass substrate thereby causing corrosion. When particularly, sodium ions are present among several alkaline components, in forming a film of a memory medium or after forming the film, difficulties are caused resulting in the worst state of deterioration of the memory medium layer or the like.
Therefore, it has conventionally been proposed, in cleaning a glass substrate immediately before a medium fabrication step to clean the glass substrate by sulfuric acid and phosphoric acid solution immediately after chemical treatment of glass to thereby remove an alkaline component on the surface (Japanese Unexamined Patent Publication No. JP-A-9-22525)
Further, although in fabricating a glass substrate having a surface roughness of about Ra 10-20 A (Angstrom), it is not so much necessary to take into consideration of the above-described presence of alkaline component, particularly, presence of sodium ions, in the case of a glass substrate having a surface roughness of Ra 5 A (Angstrom) or less that is needed in recent years, a final polishing step is necessary to provide a smaller surface roughness. However, when surface polishing is carried out on the glass substrate at a final step, in respect of the polished surface, as a result of polishing, a new surface is exposed and accordingly, the concentration of an alkaline component present on the surface of the glass substrate is not reduced. Further, when micro cracks which are fine cracks of the glass substrate are present, in the polishing step, a polishing solution or the like permeates the micro cracks and as a result, corrosion of a medium on the surface is caused in a magnetic disk composed of the glass substrate and constituting a product.
Further, even when the surface is cleaned by using normal acid or the like to remove the alkaline component on the surface after the final polishing step, in this case, although the alkaline component can be removed, the surface roughness is deteriorated by a treatment using a solution including acid. Further, a new cleaning step is necessary to remove acidic component remaining on the surface of the glass substrate. Also, when cleaning by acid is carried out, in this case, a component mainly of acid remains on the surface, or permeates into micro cracks of the glass substrate and these substances remaining on the surface or permeating into micro cracks mix into a film of a magnetic recording medium in sputtering the magnetic recording medium resulting in deteriorating the SN ratio of the medium similar to the above-described case.